Talk:Directory:Mixing Sea and River Water

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Devices based on a natural process -- when a river runs into the ocean, a huge amount of energy is unleashed because of the difference in salt concentration.

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Sewage Salinity as Power Source?

Q. What's the difference in concentration between urine and fresh water? Or urine and salt water? Could you separate out the urine and other fluids from a sewage plant and use them to generate power through this process?

A. Sea water contains about 3.5% dissolved solids, mostly sodium chloride. Urine contains about 5% dissolved solids; urea, salts and organic compounds. (I don't know how much is salts but it must be less than 3.5% as if your kidneys could make a 3.5% salt solution then you could live on seawater, and you can't.) As it comes out, urine might have a slightly higher osmotic potential than sea water but once diluted by the sewage system it wont. You can't re-concentrate the urine without putting energy in.

To get power you need two liquids with different osmotic potential, and you extract power from the tendency of their osmotic potentials to converge when they can exchange material. Here, fresh river water (resource limited by river flow) and salty sea water (you can have as much as you can pump) are used, creating less-salty sea water and saltier river water. The power is limited by the supply of the two liquids, and the osmotic potential difference between them.

As the water crosses the membrane to the salty side, it dilutes the salty water reducing its osmotic potential. Using larger quantities of salty water would limit the dilution allowing a higher pressure to be maintained. The supply of urine is rather limited compared to the supply of seawater.

Anyway, in an energy-poor world, the urea in urine probably makes it more valuable as a fertiliser. --Moth 06:49, 24 November 2009 (PST)

See also

• Directory:Hydro
• Directory:Low Impact Hydro
  • Directory:River Energy
• Directory:Ocean or Marine Power
  • Directory:Ocean Current
  • Directory:Tidal Power
  • Directory:Ocean Wave Energy
    • Directory:Wave buoys
  • Directory:Energy Islands
  • Directory:Mixing Sea and River Water
• Directory:Capillary Action Engines (un-demonstrated theory)